In order to set precisely the zero point and the sensitivity of an optical gas analyzer device, use can be made of a so-called zero gas or test gas with a known concentration, which flows through the measurement space during the calibration. As a result, the detector unit records a measured value, which is brought into correspondence with an expected intended value for calibration purposes. This known method can be used to undertake a largely precise sensitivity calibration, which removes a drift, which was, for example, caused by changes in the transmission behavior. This method is the only way to ensure the reliability of the measurement result over time. In the case of easily manageable gas components, such as carbon monoxide or carbon dioxide, this procedure can be carried out easily; in known implementations, it can be more difficult to calibrate a gas analyzer device for other gases such as nitrogen oxides, hydrochloric acid, water vapor.
The advantage of optical gas analyzer devices, on the other hand, is that many gas components can be measured at the same time. Hence, such instruments can also be suitable for emission measurements. The gas analyzer device can be checked regularly. Here, the calibration data are, in practice, checked in two steps. A reference spectrum with zero gas, for example, ambient air, is recorded regularly at short intervals, e.g., daily. This reference spectrum can be used to compensate for changes in the transmission behavior of the measurement system. Changes in the transmission behavior can, for example, be caused by changes in the radiation source or the detector, or else by contamination of the measurement chamber. The zero point is compensated for in a wavelength-dependent fashion, so that the zero point is corrected at the same time for all components.
In a further step, that can be carried out weekly to yearly, there is a regular check and, in other exemplary embodiments as desired, a calibration of the reference points for all components using a test gas. Easily manageable gases can be calibrated without additional aids using test gases from test-gas flasks. Test-gas generators are used in place of test-gas flasks in the case of gases that are difficult to manage, which test-gas generators have a relatively complex design and are difficult to operate at some locations where gas analyzer devices are used.
DE 35 22 949 A1 has disclosed an optical gas analyzer device, the calibration of which is brought about by means of test gas. The gas analyzer device operating according to the NDIR method substantially consists of a radiation source for infrared light, which passes through the one end-side window of a measurement cuvette including the measurement chamber and through the other end-side window in a comparison cuvette arranged in parallel. The measurement gas flows through the measurement cuvette, for the purposes of which an inlet port and outlet port is provided on the latter. The measurement cuvette is spatially separated from the comparison cuvette, and so the gas analyzer device has two beam paths. The light rays entering the measurement cuvette and the comparison cuvette are modulated in anti-phase by a rotating interrupter wheel. The measurement beam leaves the measurement cuvette through an output-end window and the comparison beam passes through another output-end window arranged adjacent thereto. Measurement beam and comparison beam are recorded by an opto-pneumatic detector. The front side of the detector is closed-off with the aid of an infrared-transmissive window.
Since the gas analyzer device should still supply reliable measured value, even after a relatively long time, there has to be a readjustment within the meaning of a calibration. This is brought about with the aid of an adjustment apparatus, which has a carriage arrangement, the latter holding two pairs of calibration cuvettes. While one pair of calibration cuvettes is completely filled with an inert gas, the other pair of calibration cuvettes is such that one calibration cuvette is filled with an inert gas while the other the calibration cuvette of this pair is filled with the measurement component. If carbon dioxide should be analyzed with the aid of the gas analyzer device, the measurement cuvette is filled with carbon dioxide. Said carriage arrangement can be moved to-and-fro. In the end position, the two calibration cuvettes filled with inert gas are situated in the measurement path. The zero point is calibrated in this position. If the guide arrangement is in the other end position, the other pair of unequal measurement cuvettes is situated in the measurement path. This can be used to adjust the endpoint or the sensitivity of the measurement arrangement. This solution for calibrating the measurement path is particularly suitable for cleaned carbon dioxide-free and water vapor-free test gases. However, the technical means for calibrating the measurement path, such as the various measurement cuvettes, for example, appear to be relatively complex.
Another gas analyzer device which is disclosed in DE 10 2007 065345 B3, operates on the principles of FTIR spectroscopy. Proceeding from a radiation source, a first optical system is used to generate a parallel beam by widening, which beam is incident on a semi-transparent mirror, which acts as a beam splitter. Some of the light with a fixed wavelength and frequency position, e.g., monochromatic and coherent light, is now incident on a fixed mirror and is reflected there. The other partial light beam passes through the semi-transparent mirror in a straight line and is reflected back by a movable rear mirror in the direction of the semi-transparent mirror, where these two partial light beams now interfere with one another. Here, the interference is governed in a controllable fashion by moving the rear mirror along the optical axis. From there, the interfering light passes through a measurement cuvette through which measurement gas is conducted. The interferometer achieves very precise tuning of the effective frequency position of the light beam impinging on the measurement cuvette and hence the measurement gas. Hence, a complex spectrum can be detected at the detector, and not only the absorption rate at a fixed frequency. In order to illuminate the detector optically, the split light beam is refocused by a second optical system, to be precise to the dimensions of the detector.
In order to calibrate the measurement system, it can be possible for calibration gas to be conducted through the measurement cuvette so as to reintroduce the measurement gas after the calibration step by reversing valves. As an alternative to this, a proposal suggests, optionally, to restrict the calibration gas with the aid of calibration cuvettes in the measurement path in front of the detector, to be precise for as long as the calibration or validation takes. The calibration cuvette can be thereafter pivoted out of the beam path again. The calibration cuvette is filled with a replacement gas or replacement gas mixture representing the spectrum of the measurement gas. Thus, sulfur dioxide, carbon dioxide or the like can be used as representatives of the spectral range in place of the gas components hydrochloric acid, water vapor and the like, which can be more difficult to manage.
The validation or calibration of the reference points of a gas analyzer device can, for example, if this relates to the aforementioned gases that are difficult to manage, only be carried out with great technical complexity and a high expenditure of time. This is because, additional technical equipment, such as a test-gas generator, can be installed and the gases that are difficult to manage specify a long setting time. A calibration or validation of the reference points can therefore only be carried out by educated specialists. The reference points are therefore only checked after long intervals; e.g., there is no validation of the reference points for relatively long measurement intervals. This leads to an increased risk of an erroneous evaluation of measured gas concentrations.
In this manner, it is possible to dispense with the use of a complicated test generator in the case of gases that are difficult to manage; however, it can be beneficial to provide the calibration cuvettes filled with the replacement gas.
High quality specifications are to be placed on the manufacturing process thereof in order to ensure a precise calibration. Furthermore, care has to be taken that the gases are chemically compatible amongst themselves and with the materials of the calibration cuvettes. Furthermore, the calibration cuvettes can be filled with a very high partial pressure in order to obtain a product of concentration multiplied by absorption wavelength which corresponds to that in the long path cell of an FTIR spectrometer.